The present invention relates to wire forming method for a highly integrated semiconductor device. More particularly, this invention relates to a reliable wire-forming method which prevents a protrusion from being formed within a contact hole when a capping layer is used in a semiconductor device.
Aluminum (Al) base wire material is widely adopted in forming wires in semiconductor devices. However, since the melting point of aluminum is low, its use precludes any processes subsequent to the formation of an Al wire layer from taking place at a temperature higher than 500.degree. C. In order to enable subsequent high temperature heating processes, it has been proposed to use a refractory metal having a high melting point to form a metal wire layer. However, when a refractory metal is used as the wire material and additional high-temperature (500.degree. C. to 1000.degree. C.) processes are implemented, a number of potential fabrication problems arise. For example, the refractory metal may lift off of the insulation layer it is formed on or the insulation layer may split. These problems arise due to the difference in thermal expansion coefficients between the refractory metal and the material used to form the insulation layer.
In order to eliminate these problems, the upper or lower portion of the refractory metal wire layer is capped with an nonductile insulation material such as an oxide or a nitride using a plasma enhanced chemical vapor deposition (PECVD) or a low pressure chemical vapor deposition (LPCVD) method. This nonductile insulation material suppresses the development of the stresses caused by the difference in the coefficients of thermal expansion of the refractory metal and the insulation layers.
FIGS. 1A to 1E show an example of a conventional wire forming method. Referring to FIG. 1A, a first insulation layer 12 is formed on a semiconductor substrate 10 using a ductile insulation, material such as a borophosphorus silica glass (BPSG), phosphorus silica glass (PSG), or boro silica glass (BSG). A first capping layer 14 is then formed on the first insulation layer 12 using a nonductile insulation material such as silicon nitride (Si N). Finally, a first contact hole a is formed in the first insulation layer 12 by a conventional photolithography process, exposing a first part of the semiconductor substrate 10.
Referring to FIG. 1B, titanium (Ti) or titanium nitride (TIN) is deposited on the surface of the first insulation layer 12 and the semiconductor substrate 10, including the walls of the first contact hole a, to form a barrier layer 16. Then, a refractory metal such as tungsten (W) is deposited to a predetermined thickness on the barrier layer 16, filling the contact hole a, and forming a wire layer 18.
Referring to FIG. 1 C, wire layer 18 and barrier layer 16 are then patterned by a conventional photolithography process. Next, a second capping layer 20 is formed on the whole of the resulting structure using nonductile insulation material such as silicon nitride (SiN).
Referring to FIG. 1D, additional ductile insulation material, such as BPSG, PSG or BSG, is deposited on the surface of the second capping layer 20 to form a second insulation layer 22.
Referring to FIG. 1E, the second insulation layer 22, the first and second capping layers 14 and 20, and the first insulation layer 12 are then etched by a conventional photolithography process, to form a second contact hole b which exposes a second part of the semiconductor substrate 10.
According to this method, the first and second capping layers 14 and 20 are formed of a hard, nonductile material that encloses the wire layer 18, preventing the wire layer from lifting off of the first insulating layer 12 and preventing the first insulating layer 12 from splitting.
However, when the second contact hole b is formed using a conventional photolithography, a protrusion (portion c in FIG. 1E) is produced in the second contact hole b. This protrusion results from the difference in the etch rates of first and second insulation layers 12 and 22, which are formed of a ductile insulation material, and etch rates of the first and second capping layers 14 and 20, which are formed of a nonductile insulation material. The existence of this protrusion c serves to lower the reliability of the wire-forming process of the semiconductor device.